1. Field of the Invention
The present invention relates to a midsole and the method for manufacturing it. More particularly, the present invention relates to a method and a midsole which has three dimensional peripheral component having a shell filled with foam material and providing wear-resistant surface with precise ornamental details.
2. Description of the Related Art
Midsoles for athletic shoes have been characterized as the most important part of footwear. This portion of the shoe is the interface between the foot of the wearer and the ground. The rather significant force at which the foot of a runner strikes the ground, especially at the toe and heel portions, is transmitted through the body of runner. The force developed is absorbed in significant part by the midsole.
The increase in demand for shoes for sports and outdoor activities such as walking, running, hiking, tennis, basketball and numerous other high activity sports have prompted many advances in shoe design to provide improved protection and comfort to the feet, ankles, legs, hips, etc. Efforts to improve shoes have entered on decreasing shoe weight and improving cushioning, flexibility, and stability.
The midsole contributes to foot stability and is the primary shock absorption member of the sole. The midsole is composed generally of a softer, more flexible material than the outsole. Since the midsole is important to such factors as stability and shock absorption, the design of the midsole has received considerable attention by sport shoe manufacturers.
A foam midsole material by itself is generally inadequate to provide the stability and cushioning demanded for modern sport shoes. The foams used in current soles have insufficient external surface tension by themselves to provide the required stabilizing forces in response to pressures exerted on a sole.
Several molding process for fabricate a midsole are described in U.S. Pat. Nos. 4,730,402 and 4,876,053, both of which are issued to Norton et al. and both of which are hereby incorporated by reference. Furthermore, the process of molding EVA with polyurethane is also disclosed in U.S. Pat. Nos. 4,551,930, 4,561,140, and 4,654,983, all issued to Graham et al. and all of which are hereby incorporated by reference.
While there are advantages gained by compression molding a plastic material, e.g. extending the life of the material, there are attendant disadvantages of such compressive forces. In particular, the compressive force is substantially parallel to the inner surface of the sidewall of the mold. Consequently, the outer surface of the performed receives very little lateral compressive forces. Instead, the outer surface of the preform aligned along the inner surface of the mold receives mostly vertically directed compressive forces. Consequently, it is very difficult to obtain ornamental details on the sidewalls of the midsole which have a high resolution.
In order to develop fine details on the outer surface of the midsole, it is necessary to apply the compressive forces in a direction which is aligned substantially perpendicular with respect to the peripheral surface of the midsole.